Cyclophosphazene fluids are known to be useful as lubricants, hydraulic fluids, fuel additives, flame retardants, and for a variety of other purposes as well. The polyfluoro-substituted cyclophosphazenes, especially the polyfluoro-substituted cyclotriphosphazenes, appear to be a particularly useful group of cyclophosphazenes. As a class, the cyclophosphazenes appear to have significant potential as high temperature lubricants or hydraulic fluids, that is, at service temperatures in excess of 250.degree. C. Applications at these high temperatures include use in jet aircraft, turbine and diesel engines.
Unfortunately, when employed at high temperatures in ambient atmospheres, the cyclophosphazenes can oxidize, and their utility thereby suffer. Oxidation of the cyclophosphazenes can cause the formation of solid deposits in them, can change their viscosities or lubricities, and can increase their corrosivity (that is, increase their acid number). U.S. Pat. No. 4,724,264 (Feb. 9, 1988, Nakacho et al.) discloses an attempt to avoid the problem of oxidation by employing particular substituents on the cyclophosphazene which render it less subject to oxidation. However, the selection of these particular substituents defines and therefore limits the circumstances under which the substituted cyclophosphazene fluid may be used.
At least one attempt has been made to protect cyclophosphazene fluids from oxidation by adding an antioxidant to them. More particularly, U.S. Pat. No. 3,313,731 (Apr. 11, 1967, Dolle, Jr. et al) discloses the use of perfluorinated aryltin compounds as antioxidants in cyclic triphosphonitriles (cyclotriphosphazenes) and polyfluoroalkoxy-substituted triazines. Of course, the triaryltin compounds are now known to be among the leading high temperature antioxidant additives for other conventional fluids, such as the polyphenyl ether fluids.
The '731 patent discloses good utility for the perfluorinated aryltins in the triazine fluids. However, the data given in the patent also disclose that less success was enjoyed when the aryltins were employed in the cyclotriphosphazenes. Comparative Sample A disclosed in Example below further demonstrates that the perfluorinated triaryltins are less than satisfactory as antioxidants in cyclophosphazenes, because the protection they provide is inadequate for high temperature uses of the cyclophosphazenes. Indeed, the use of aryltin compounds has often been found to be detrimental to the properties of the cyclophosphazenes.
Other antioxidant materials are of course known to be useful in other fluids. For example, it has long been known to employ triarylphosphines and phosphine oxides as antioxidant or anticorrosive additives in perfluorinated aliphatic polyether or polyol ester lubricants. The triarylphosphines and other known antioxidants, however, would be expected to lack utility in cyclophosphazene fluids for a variety of reasons. The known antioxidants are often immiscible with cyclophosphazene fluids. Known antioxidants often have a high volatility, such that they do not remain in cyclophosphazene fluids very long when employed at high temperatures. Many known antioxidants also possess low thermal stability, again, rendering them less than useful for high temperature applications. Some antioxidants have unacceptable decomposition products, interfering with the desired function of cyclophosphazene fluids. Similarly, some known antioxidants have an unacceptable coefficient of viscosity at high temperatures, that is, they interfere with the viscosity or lubricity of cyclophosphazene fluids. Indeed, many known antioxidants have narrow liquid ranges, that is, they have a high pour point and a low boiling point.
Perhaps the most significant reason why the antioxidants useful in other fluids are not expected to be useful in cyclophosphazene fluids is that cyclophosphazene fluids have an oxidative degradation mechanism which is substantially different from those of conventional fluids or lubricants, such as the polyphenyl ethers, the polyol esters and the perfluorinated aliphatic polyethers. Put simply, if the known antioxidants work by interfering with one step or another in the oxidative pathway associated with the oxidation of a conventional fluid, and if the oxidative pathway for cyclophosphazenes lacks such a step, the conventional antioxidants couldn't be expected to interfere in the oxidative pathway for the cyclophosphazenes.
As a more particular example, it is known that antioxidants for polyphenyl ether fluids function by the formation of a free radical, which reacts in a series of reactions with compounds such as the phenols, in order to form more stable radicals. As explained in more detail in "High-Temperature Stabilization of Polyphenyl Ethers By Inorganic Salts," Ravner et al., 15 ASLE Transactions 45-53 (1971), the antioxidants act as electron sinks during the free radical reaction, and the overall oxidation rate of the base fluid is thus curtailed by the antioxidants. The paragraph bridging pages 50 and 51 of the article notes a variety of potential reaction pathways in the neat ether fluids. The article, and in particular that paragraph, are expressly incorporated by reference herein.
In contrast, oxidation of cyclophosphazene fluids does not entail a free radical reaction. Rather, it appears that cyclophosphazenes oxidize by a cationic mechanism, such that the major oxidation products are arylphosphate esters, arylphosphate ester acids, arylphosphate ester amides, and arylphosphate ester nitriles. The complete oxidation mechanism of the cyclophosphazenes is not yet understood, and research into the mechanism is continuing. However, the cationic mechanism for oxidation of cyclophosphazenes may proceed as follows: ##STR2##
Whatever the details of the true mechanism may be, it is clear that conventional antioxidants such as organometallic salts (for example, triaryltins), being electron sinks, could not reasonably be expected to interfere in the oxidation of the cyclophosphazenes, since their oxidation entails a cationic mechanism. Despite the resulting expectation that conventional antioxidants are not useful in cyclophosphazenes, it would still be highly desirable to find materials which could successfully function as antioxidants in cyclophosphazenes.
It is therefore an object of the present invention to provide a cyclophosphazene fluid with an effective antioxidant, so as to stabilize the cyclophosphazene fluid against oxidation when used in ambient air at the high operating temperatures encountered in jet aircraft, turbine and diesel engines.
It is another object of the present invention to provide cyclophosphazene fluids with antioxidants having relatively low volatility, high thermal stability, tolerable decomposition products and a wide liquid range, that is, a low pour point and a high boiling point.